Surface acoustic wave (SAW) devices used as sensors in measurement system are known, For example, a tire pressure monitoring system (TPMS) helps to avoid accidents by warning the driver about tire pressure problems. TPMS is a vehicle-embedded system detecting the tire pressure by analyzing the difference between the wheel speeds or by measurement of pressure and temperature. System like a direct TPMS system typically consists of one UHF receiver in the vehicle and four sensors mounted on the wheel rim or valve to sense data, to calibrate pressure versus temperature and to organize data transmission to the car body.
Various other SAW sensor applications are known in the art. In particular, many different techniques have been proposed for sensing the temperature of a component in an industrial process or system. Pressure, as with use in tires and for delivering this information to the operator at a central location on the vehicle, can be used in industrial system to convey pressure differentials during processing operations (e.g., dairy, petroleum, medical, aeronautical, deep sea, etc., applications).
The majority of prior art sensors are direct active systems, some utilizing a silicon micro-electro-mechanical system (MEMS) based sensor powered by a battery. Where several sensor are utilized throughout a target system, pressure and temperature information is transmitted by radio from each sensor locations (e.g., each of the wheels on a motor vehicle) to an electronic control unit (ECU) and displayed as either a number or a warning indicator. The problem associated with using such prior art systems in, for example, a TPMS environment is that the need to remove the tire for access to the batteries, and the need to rebalance the tires after battery replacement, together with the disposal of worn out batteries are the major shortcomings of direct sensing systems. Batteries inside tires add weight, have limited life and cannot be replaced. Furthermore, they require some sort of electrical connection between the sensor and any remote monitoring device. With a rotating wheel, this electrical connection requires special contacts, complicating the system, introducing added cost and reducing reliability.
Conventional wireless systems are not durable and are expensive to design and produce. The sensors and transmitters must also be able to withstand the harsh environment, such as when used inside a vehicle tire that includes high temperatures, shock and vibration, and centrifugal forces from tire rotation. Although it has the advantage of wireless communication of the pressure to a remotely placed monitor, it is difficult to install and service, and requires special adaption of the wheel.
One particular type of sensor, or condition-responsive device, which has recently become desirable for use in certain electronics systems, is an acoustic wave device, such as a surface acoustic wave (SAW) device. SAW devices have desirable properties for certain sensor applications since they are sensitive, use very little power, and can be operated at radio frequencies convenient for relaying information in a wireless fashion. SAW devices may include at least one resonator element made up of interdigitated electrodes deposited on a piezoelectric substrate. One of the problems with current SAW sensor designs, particularly those designs adapted to tire pressure and temperature sensing applications, is the inability of conventional SAW sensing systems to meet the rigorous environment within the environment itself. Such systems are inherently expensive, awkward, and often are not reliable in accurately sensing at least one of tire air pressure and temperature.
A need therefore exists for an improved wireless and batteryless SAW sensor apparatus and packaging system, which for example can be integrated into a tire and interrogated wirelessly, and that the sensors are ultimately more efficient and sturdier than presently implemented sensors. Such an apparatus is described in greater detail herein.